1. Field
This disclosure relates to power supplies having high inrush currents incurred during initial AC turn on and during power cycling.
2. Description of the Related Art
The charging of a bulk capacitor during start-up of an off-line AC/DC power supply generates a surge current also known as an inrush current. The impact of inrush current can be mitigated in various ways. The techniques of reducing the impact of an inrush current are often referred to as inrush limiting and are performed by inrush limiters and inrush limiter circuits.
In low to medium power switching power supplies, inrush limiting may be accomplished by a negative temperature coefficient thermistor. In higher power switching power supplies, a variety of approaches have been utilized in an attempt to improve the efficiency of the main power circuit. In one approach, the current limiting element is bypassed after a bulk capacitor is charged. In one approach, a silicon controlled rectifier (SCR) lockout circuit is enabled so that main power flows around the inrush limiter rather than through it.
Some inrush limiting solutions have been accomplished with resistive elements that dissipate power. The watt-seconds dissipated in a resistive inrush limiter is equal to the watt-seconds stored in the bulk capacitor when it is charged to the peak voltage of the AC line. Using resistive inrush limiting is typically successful for a single turn on event when the resistor has a high surge rating. However, if the line keeps recycling, then the resistive inrush limiter suffers high dissipation and failures could occur.